Microfluidic device and method for focusing, segmenting, and dispensing of a fluid stream

ABSTRACT

A microfluidic device for forming and/or dispensing minute volume segments of a material is described. In accordance with one aspect of the present invention, a microfluidic device and method is provided for spatially confining the material in a focusing element. The device is also capable of segmenting the confined material into minute volume segments, and dispensing a volume segment to a waste or collection channel. The device further includes means for driving the respective streams of sample and focusing fluids through respective channels into a chamber, such that the focusing fluid streams spatially confine the sample material. The device may also include additional means for driving a minute volume segment of the spatially confined sample material into a collection channel in fluid communication with the waste reservoir.

CLAIM FOR BENEFIT OF PRIORITY

[0001] This application claims the benefit of priority of copendingprovisional application No. 60/175,684 filed Jan. 12, 2000 the entiretyof which is incorporated herein by reference.

GOVERNMENT RIGHTS STATEMENT

[0002] The U.S. Government has a paid-up license in this invention andthe right in limited circumstances to require the patent owner tolicense others on reasonable terms as provided for by the terms ofcontract No. DE-AC05-00OR22725 awarded by the U.S. Department of Energy.

FIELD OF THE INVENTION

[0003] This invention relates to microfluidic devices for theelectrokinetic manipulation of fluidic chemical and biologicalmaterials. More specifically, this invention provides a microchip devicefor spatially confining and dispensing a sample material stream in amanner which lowers the axial extent of the dispensed plug.

BACKGROUND OF THE INVENTION

[0004] In order to facilitate the development of the biological andchemical sciences, fluidic microchip technologies are increasinglyutilized to perform traditional chemical laboratory functions within acontrolled microfabricated environment. Microfabricated chemicalinstrumentation, also known as “lab-on-a-chip” technology, requires thedevelopment of a plurality of microfabricated functional elements orunit processes cooperatively linked on the microchip to perform smallvolume chemical and biochemical measurements.

[0005] These “on-chip” laboratories facilitate the precise transport andanalysis of fluidic chemical and biological materials. The knownmicrofluidic devices provide the advantages of reduced analysis time andreagent consumption, ease of automation, and valveless fluid control ofnanoliter to sub-nanoliter volumes. A variety of electrically drivenseparations have been performed within microchannel networks. Microchipshave also been developed for controlling chemical reactions, includingarrays for solid-phase chemistry, reaction wells for polymerase chainreactions, channels with immobilized enzymes for flow injectionanalysis, and manifolds for homogenous enzyme assays.

[0006] The ability to design and machine channel manifolds withlow-volume connections renders microchips suitable for performingseveral steps of an analytical process on a single device. Microchipsthat perform multiple chemical reactions with the speed of microscale CEanalysis have been fabricated for pre- and post-separation reactions,for DNA restriction digests with fragment sizing, and for cell lysis,multiplex PCR amplification and electrophoretic sizing.

[0007] Electrokinetic techniques, i.e., electroosmotically induced fluidtransport and/or electrophoretic migration of ions, are the preferredmethods of manipulating biological and chemical materials onmicrofluidic devices. The mixing of two or more liquid-phase materialsor the dispensing of a reagent material on a microchip is accomplishedby controlling the electric potentials applied to the various reservoirsto electrokinetically drive the materials contained therein through thechannels of the microchip. Electrophoresis transports charged species,whereas electroosmosis imparts a velocity to all ions and neutralspecies. Under conditions where both electroosmosis and electrophoresisare operative, the net velocity of an ion will be the vector sum of theelectroosmotic and electrophoretic velocities.

[0008] Electrokinetic transport mechanisms are highly effective foreffectuating a number of highly useful experiments as identified above.Several applications require the ability to spatially confine a samplematerial stream with consistent reproducibility. This spatialconfinement or “electrokinetic focusing” refers to the use ofelectrokinetic transport to confine spatially the transport of bothfluids and ions. An example of such focusing is disclosed in relatedU.S. Pat. No. 5,858,187, issued on Jan. 12, 1999, which describes andshows a microfluidic device and method for spatially confining a streamof fluidic material.

[0009] Further applications require the ability to dispense a volumesegment of a sample material with consistent reproducibility. An exampleof such dispensing is disclosed in U.S. Pat. No. 5,858,195, granted Jan.12, 1999, which describes and shows a microfluidic device and method fordispensing a volume segment of a sample material. The entire disclosureof said U.S. Pat. No. 5,858,195 is incorporated herein by reference.

[0010] More recently, a need has arisen for an improved microchipwherein the profile of the dispensed segment may be controlled todispense more minute quantities. Examples of the benefit of shorteraxial extent material segments include (i) decreasing the lengthrequired for a separation and reducing the analysis time, (ii) enablingfaster axial mixing by diffusion of the segment with adjacent materials,and (iii) increasing the number of material segments per unit axiallength of channel.

SUMMARY OF THE INVENTION

[0011] In accordance with one aspect of the present invention there isprovided a fluidic microchip having a microchannel network for thespatial confinement of a material stream. The microchannel networkincludes a focusing element formed in a surface of a substrate. A samplechannel is formed in the surface of the substrate for conducting a firstsample fluid stream therethrough. The sample channel has a first end influid communication with a source of a sample fluid and a second end influid communication with the focusing element. A focusing channel isalso formed in the surface of the substrate for conducting a focusingfluid stream therethrough. The focusing channel has a first end in fluidcommunication with a source of focusing fluid and a second end in fluidcommunication with the focusing element. A sample-waste channel formedin the surface of the substrate has a first end in fluid communicationwith the focusing chamber and a second end in fluid communication with asample-waste reservoir. The fluidic microchip further includes means fordriving the respective streams of the sample and focusing fluids throughthe respective channels into the focusing chamber, whereby the focusingfluid stream spatially confines the sample fluid stream within thefocusing chamber.

[0012] In accordance with a second aspect of the present invention thereis provided a fluidic microchip that is adapted for the spatialconfinement of an electrokinetically driven fluidic material stream topermit dispensing of a low profile volume segment of the fluidicmaterial. The apparatus includes a focusing element and a sample channelformed in the surface of a substrate for carrying a sample fluid streamtherethrough. The device also includes a focusing channel for carrying afocusing fluid stream therethrough. In addition, the device includes asample-waste channel for carrying a focused stream of the sample fluidstream therethrough. Further, the device includes a buffer channel forcarrying a buffer fluid stream therethrough. Still further, the deviceincludes a collection channel for carrying a volume segment of thefocused stream of the sample fluid therethrough.

[0013] The sample channel, focusing channel, collection channel, bufferchannel, and sample-waste channel have respective first ends in fluidcommunication with a source of sample fluid, a source of focusing fluid,a source of buffer fluid, a reservoir for sample waste fluid, and areservoir for a volume segment of the sample fluid stream, respectively.The sample, focusing, buffer, sample-waste, and collection channels havetheir respective second ends in fluid communication with a chamber.

[0014] The buffer, sample, sample-waste, and collection channelsintersect directly to form a valving element. The buffer channel andcollection channel are formed such that they are positioned opposite toeach other across the valving element. The focusing channel intersectsthe sample channel ahead of and adjacent to the valving element. Thefocusing channel is formed such that it is positioned more proximal tothe sample fluid source than either the buffer channel or collectionchannel.

[0015] The apparatus may include electromotive means operativelyconnected to the sample fluid and the focusing fluid for (i)electrokinetically driving their respective streams of the sample andfocusing fluids through their respective channels into said focusingelement such that the focusing fluid stream spatially confines thesample fluid stream within the chamber and (ii) electrokineticallydriving the stream of the buffer fluid through the respective channelinto said valving element chamber such that a volume segment of thefocused sample stream is dispensed into and transported along thecollection channel.

[0016] In accordance with another aspect of the present invention thereis provided a fluidic microchip having each of the features of theaforementioned first aspect of the invention with the addition of asecond focusing channel. In this aspect of the device, two focusingchannels are formed in the surface of said substrate for carryingfocusing fluid streams therethrough. The focusing channels haverespective first ends in fluid communication with a source of thefocusing fluid and respective second ends in fluid communication withthe focusing element. The focusing channels are formed such that theyare positioned on opposite sides of the focusing element. The focusingchannels are further formed such that they are positioned more proximalto the sample fluid source than either the buffer channel or collectionchannel.

[0017] In accordance with yet another aspect of this invention, there isprovided a method for dispensing minute volume segments of the focusedsample stream. The method includes the step of conducting a stream of asample fluid through a sample fluid channel to a focusing element. Inthe conducting step the width of the sample fluid stream is narrowed inthe focusing chamber by transporting a focusing fluid into the focusingelement on one side or two opposite sides of the sample fluid stream.The method includes the further step of withdrawing a portion of thefocused sample stream from the focusing chamber into the respectivesample and sample-waste channels. A stream of buffer fluid is conductedthrough a buffer channel into the valving element such that a volumesegment of the focused sample stream is dispensed into and transportedalong the collection channel.

[0018] Here and throughout this application, the term “chamber” is usedto describe an area or zone that includes a “focusing element” and a“valving element” as those terms are described herein.

[0019] An alternative to electrokinetic transport mechanisms, the movingof the sample and buffer materials, and any other materials used in adevice or method according to the present invention can be accomplishedby application of pressure or vacuum to the appropriate channel orchannels. It is also contemplated that combinations of electrokinetic,pressure, and/or vacuum transport mechanisms can be utilized inimplementing a given device or method in accordance with this inventionif desired.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The foregoing summary, and the following detailed description,will be better understood when read in conjunction with the attacheddrawings, in which:

[0021]FIG. 1A is a schematic diagram of a fluidic microchip channelnetwork for spatially focusing sample material streams in accordancewith a first embodiment of the present invention;

[0022] FIG. B is a schematic diagram of a fluidic microchip channelnetwork for spatially focusing sample material streams in accordancewith a second embodiment of the present invention;

[0023]FIGS. 2A and 2B are enlarged views of the chamber of the fluidicmicrochip device shown in FIG. 1A;

[0024]FIGS. 3A and 3B are enlarged views of the chamber of the fluidicmicrochip device shown in FIG. B;

[0025]FIG. 4 is a schematic view showing electrical connections betweenvarious reservoirs of the microchip and a voltage source;

[0026]FIG. 5A is a white light image of a chamber used in a fluidicmicrochip according to the present invention as depicted schematicallyin FIG. 1B; and

[0027]FIG. 5B is a fluorescence image of the chamber of FIG. 5A showingthe focusing of a fluid stream therein.

DETAILED DESCRIPTION

[0028] The fluidic microchip according to this invention will bedescribed in connection with two preferred embodiments thereof. In oneembodiment, a focusing element is provided on the microchip in which asample material stream is laterally confined or focused using a singlefocusing fluid stream provided through a first focusing channel. In thesecond embodiment, the microchip according to this invention includes asecond focusing channel for providing a second focusing fluid streamoperating in conjunction with the first focusing fluid stream forlaterally confining or focusing the sample material stream. In both thefirst and second embodiments, the microchip according to this inventionprovides a collection channel for receiving a volume segment of thefocused sample stream, wherein the volume segment of the focused samplestream is generated by transporting a buffer fluid from a buffer channelthrough the valving element and into a collection channel.

[0029] A device embodying this invention preferably uses electrokineticforces to drive streams of fluidic materials through the microfabricatedchannels. The microfabricated device includes a sample channel and oneor two focusing channels in fluid communication with a focusing elementfor spatially confining the streams of fluidic materials travelingtherethrough. The microfabricated device further includes buffer andcollection channels in fluid communication with the valving element forconfining, segmenting, and transporting volume segments of the confinedstream of sample fluidic material. The combination of the focusingelement and the valving element defines a chamber within which thefocusing and dispensing functions take place.

[0030] The sample and focusing fluid streams are manipulated such thatthe sample fluid stream can be narrowed to a desired width. Focusing ofthe sample fluid stream is preferably achieved using electrokinetictechniques such as electrophoresis, electroosmosis, or a combinationthereof. The focusing or spatial confinement is achieved by using afluidic buffer material that is transported to the focusing elementthrough the focusing channels. The spatial confinement of the samplefluid stream passing through the focusing element is caused by havingconfluent flow of the focusing fluid with the sample fluid.

[0031] Referring now to the drawings, and in particular to FIGS. 1A and2A, there is shown schematically a fluidic microchip device 30 inaccordance with the present invention. Fluidic microchip device 30 isdesigned and fabricated from a solid substrate material, preferablyglass. Silicon may also be used because of the well-developed technologypermitting its precise and efficient fabrication. Other materials,including polymers, quartz, fused silica, sapphire, or plastics are alsosuitable as substrate materials. The surface of the fluidic microchipdevice 30 is covered and sealed by a cover plate in a known manner asdisclosed in U.S. Pat. No. 5,858,187, the entirety of which isincorporated herein by reference.

[0032] The fluid conduction channels on fluidic microchip device 30 areformed using micromachining methods known to those skilled in the art.The micromachining methods available include film deposition processessuch as spin coating, chemical vapor deposition, laser fabrication,photolithographic techniques such as UV or X-ray processes, or etchingmethods which may be performed by either wet chemical processes orplasma processes. Preferably, the microchannel configurations aretransferred onto the substrate material using a positive photoresist,photomask, and UV exposure. The channels are etched into the substratein a dilute, stirred BF/NH₄F bath.

[0033] The fluidic microchip device 30 has a sample channel 46, afocusing channel 50, a buffer channel 40, a sample-waste channel 42 anda collection channel 44 formed in the surface thereof, which intersectat common ends to form a chamber 52 which is defined by a focusingelement 54 and a valving element 56. The focusing channel 50 intersectswith the sample channel 46 to form the focusing element 54. The samplechannel 46, sample-waste channel 42, buffer channel 40, and collectionchannel 44 intersect to form the valving element 56. Sample reservoir 38is formed or positioned at the end of the sample channel 46, such thatthe sample channel 46 is in fluid communication with the reservoir 38.Focusing reservoir 48, buffer reservoir 32, sample-waste reservoir 34,and waste reservoir 36 are similarly associated with the focusingchannel 50, buffer channel 40, sample-waste channel 42, and collectionchannel 44, respectively.

[0034] An electric voltage source 31 is connected to the variousreservoirs through platinum wires 33 that extend into the reservoirs asshown in FIG. 4. Electrical contact to the reservoirs is made usingplatinum wire to operably link the fluidic microchip device 30 with thesource of electric potential.

[0035] In the operation of the fluidic microchip device 30 in FIGS. 1Aand 2A, the sample fluid is infused through the focusing element 54 ofchamber 52. The electric potentials at the reservoirs are adjusted toincrease or diminish the degree of lateral focusing. The potentialsapplied at the sample and focusing reservoirs 38 and 48 are controlledindependently, and the sample-waste reservoir 34 is preferably grounded.Small variations in channel lengths and widths may require that thevoltages applied to the reservoirs differ slightly for balancing thefield strengths to obtain a symmetric fluid transport in the focusingchamber 52. Assuming that the operational conditions, e.g., relativeconductivities of the buffers, applied voltages, electroosmotictransport, remain unchanged, the stream width over time remainsessentially constant.

[0036] Lateral spatial confinement (focusing) of the sample materialstream from sample channel 46 occurs with the transport of focusingfluid into the focusing element 54 from the focusing channel 50 and thetransport of sample fluid into focusing chamber 52 from sample channel46. If the focusing fluid flow is too large, the sample is notelectrokinetically transported from the sample reservoir 38 to thefocusing chamber 52. In FIG. 2A, arrows depict flow directions in thevarious channels during focusing.

[0037] Dispensing of the focused sample material stream from chamber 52is shown in FIG. 2B. Dispensing begins with the application of electricpotentials at the sample reservoir 38, focusing reservoir 48, and thesample-waste reservoir 34 such that the sample fluid is transported fromthe sample channel 46 and the sample-waste channel 42 towards reservoirs38 and 34, respectively. Simultaneously, or nearly simultaneously,electric potentials are applied to the buffer reservoir 32 and wastereservoir 36 causing the buffer fluid from the buffer reservoir 32 toflow through the buffer channel 40 and into the collection channel 44. Aportion of the segment 49 of the focused sample stream that is presentin the intersection between buffer channel 40 and collection channel 44is transported into collection channel 44 by movement of the bufferfluid. In FIG. 2B, arrows depict flow directions in the various channelsduring dispensing.

[0038] A second preferred embodiment of a focusing and dispensing deviceaccording to the present invention will now be described with referenceto FIGS. 1B, 3A, and 3B. A fluidic microchip 60 has a first focusingchannel 84, a sample channel 82, a sample-waste channel 76, bufferchannel 74, a collection channel 78, and a chamber 86 which are formedand disposed on a substrate in the same manner as the embodimentdescribed above. Each of the aforementioned respective channels is influid communication with the respective reservoir in the same manner asthe first embodiment. However, the fluidic microchip 60 also has asecond focusing reservoir 68 and a second focusing channel 80, which isin fluid communication with the chamber 86. The chamber 86 is defined bya focusing element 88 and a valving element 90. The focusing channels80, 84 intersect with the sample channel 82 to form the focusing element88. The sample channel 82, sample-waste channel 76, buffer channel 74,and collection channel 78 intersect to form the valving element 90.

[0039] In the operation of the fluidic microchip device 60, the samplefluid is infused through the focusing chamber 86. The electricpotentials at the reservoirs are adjusted to increase or diminish thedegree of lateral focusing. The potentials applied at the sample andfocusing reservoirs are controlled independently, and the sample-wastereservoir 64 is preferably grounded. Small variations in channel lengthsand widths may require that the voltages applied to the reservoirsdiffer slightly for balancing the field strengths to obtain desiredfluid transport in the chamber 86. Assuming that the operationalconditions, e.g., relative conductivities of the buffers, appliedvoltages, electroosmotic transport, remain unchanged, the variability ofthe stream width over time remains essentially constant.

[0040] Focusing of the sample material stream from sample channel 82occurs with the transport of focusing fluid into the focusing element 88from the focusing channels 80 and 84 and the transport of sample fluidinto focusing chamber 86 from sample channel 82. When the fluid flowrates in each focusing channel 80 and 84 are equal, the sample materialstream is confined substantially along the center axis of the samplechannel 82. The position of sample material stream within the samplechannel 82 may be controlled by adjusting the ratio of fluid flow ratesbetween the two focusing channels 80 and 84. If the focusing flow is toolarge, the sample is not transported from the sample reservoir 20 to thechamber 86. In FIG. 3A, the arrows depict flow directions in the variouschannels during focusing.

[0041] Dispensing of a volume segment of the focused sample materialstream from chamber 86 is shown in FIG. 3B and proceeds in a similarmanner to the dispensing process using in the embodiment depicted in ofFIG. 2B. In FIG. 3B, the arrows depict flow directions in the variouschannels during dispensing.

WORKING EXAMPLE

[0042] In order to demonstrate the functionality of the apparatus andprocess according to the present invention, a microchannel device havinga microchannel network as shown schematically in FIG. 1B was fabricated.FIG. 5A shows a white light image of the chamber 86 schematicallydepicted in FIGS. 1B, 3A, and 3B with sample channel 82, focusingchannels 80 and 84, buffer channel 74, collection channel 78, andsample-waste channel 76. In the device shown in FIG. 5A the channels are9.0 μm deep and 42 μm wide. The buffer channel 74 and first focusingchannel 84 are separated by 14,5 μm. The channel lenghts, in millimeters(mm), for this miscrochip divice are listed in Table 1 below. Theelectroosmotic flow in this experiment it that of native glass. TABLE 1Channel Length 74 14.8 76 3.9 78 15.6 80 15.9 82 6.2 84 16.0

[0043] Focusing of the sample material stream is shown in thefluorescence image in FIG. 5B. Rhodamine in sodium tetraborate buffer isthe sample and sodium tetraborate buffer is the buffer and focusingfluid. Focusing of the sample material stream from sample channel 82occurs with the flow of focusing fluid into the chamber 86 from thefocusing channels 80 and 84 and sample fluid into the chamber 86 fromsample channel 82. The fluid flow rates in focusing channels 80 and 84are approximately equal, and the sample material stream is confinedsubstantially along the center axis of the chamber 86. In FIG. 5B,arrows depict direction of flow in chamber 86. The fluid flow from thebuffer channel 74 and the collection channel 78 during focusing helpsexpand, maintain, or further confine the sample stream depending on thepotential applied to the respective reservoirs 62 and 66. Set forth InTable 2 are the electric potentials in kilovolts (kv) applied to thereservoirs during focusing of the sample fluid (Focusing) and dispensingof a volume segment of the sample fluid (Dispensing). TABLE 2 ReservoirFocusing Dispensing 62 1.9 1.9 64 0 0.38 66 1.9 0 68 1.9 0.38 70 1.30.38 72 1.9 0.38

[0044] To obtain a narrower sample stream width, higher potentials canbe applied to the focusing reservoirs 68 and 72 relative to thepotential at the sample reservoir 70. Similarly, higher potentials canbe also applied to the buffer reservoir 62 and waste reservoir 66relative to the potential at the sample reservoir 70. Adjustments of theelectric potentials at the focusing reservoirs 68 and 72, bufferreservoir 62, and waste reservoir 66 can also be made simultaneously toadjust the stream width. Conversely, lower potentials relative to thepotential at the sample reservoir 70 will result in a wider samplestream. FIG. 6 shows the integrated profile of a portion of the segment49 of the sample stream after being dispensed into the collectionchannel 78. The profile was detected 0.1 mm downstream from the chamberin the collection channel 78 with laser induced fluorescence.

[0045] Microfabricated devices in accordance with the present inventionhave been described. The devices employ focusing for spatially confininga stream of sample material passing therethrough. The device furtheremploys transport of fluidic material to segment and dispense a portionof the focused sample material stream. The combination of focusing anddispensing is novel. In this way, the microfabricated device is capableof delivering precise, minute volumes of fluidic material reproducibly,by adjusting the axial extent of the dispensed plug. The device of thepresent invention is advantageous for use in various analyticalprocedures such as chemical separations. The spatial confinement of thesample fluid in and dispensing of controlled minute volumes from thechamber allows high efficiency, high sample throughput. Also, amicrofluidic device in accordance with this invention can be operatedwith any combination of electrokinetic transport and pressure-drivenflow to achieve comparable results.

[0046] The terms and expressions which have been employed are used asterms of description and not of limitation. There is no intention in theuse of such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof. It is recognized,however, that various modifications such as channel dimension, location,and arrangement are possible within the scope of the invention asclaimed.

What is claimed is:
 1. An apparatus for the spatial confinement of amaterial stream, comprising: a chamber formed in a surface of asubstrate; a sample channel formed in the surface of the substrate forconducting a sample fluid stream therethrough, said sample channelhaving a first end in fluid communication with a source of a samplefluid and a second end in fluid communication with said chamber; afocusing channel formed in the surface of the substrate for conducting afocusing fluid stream therethrough, said focusing channel having a firstend in fluid communication with a source of focusing fluid and a secondend in fluid communication with said chamber; a waste channel formed inthe surface of the substrate, said waste channel having a first end influid communication with said chamber and a second end in fluidcommunication with a waste reservoir; and means for driving therespective streams of the sample and focusing fluids through therespective channels into said chamber, whereby the focusing fluid streamspatially confines the sample fluid stream within said chamber.
 2. Anapparatus as set forth in claim 1 further comprising a buffer channelformed in the surface of the substrate for conducting a buffer fluidstream therethrough, said buffer channel having a first end in fluidcommunication with a source of a buffer fluid and a second end in fluidcommunication with said chamber; and means for driving the buffer fluidthrough said buffer channel into said chamber such that the buffer fluidacts on the spatially confined sample fluid stream.
 3. An apparatus asset forth in claim 2 further comprising a collection channel formed inthe surface of the substrate for conducting a fluid stream therethrough,said collection channel having a first end in fluid communication withsaid chamber and a second end in fluid communication with a wastereservoir, said collection channel being adapted to conduct the bufferfluid from said buffer channel; and means for driving the buffer fluidin said collection channel into said chamber such that the buffer fluidacts on the spatially confined sample fluid stream.
 4. An apparatus asset forth in claim 1 wherein said means for driving the streams of thesample and focusing fluids through the respective channels into saidchamber comprises electromotive means for providing electrokinetictransport of the sample and focusing fluids through their respectivechannels.
 5. An apparatus as set forth in claim 2 wherein said means fordriving the streams of the sample and focusing fluids through therespective channels into said chamber and said means for driving thebuffer fluid through said buffer channel into said chamber compriseelectromotive means for providing electrokinetic transport of thesample, focusing, and buffer fluids through said sample, focusing, andbuffer channels, respectively.
 6. An apparatus as set forth in claim 3wherein said means for driving the streams of the sample and focusingfluids through the respective channels into said chamber, said means fordriving the buffer fluid through said buffer channel into said chamber,and said means for driving the buffer fluid and the spatially confinedsample fluid stream through said collection channel compriseelectromotive means for providing electrokinetic transport of thesample, focusing, buffer, and spatially confined sample fluids throughsaid sample, focusing, buffer, and collection channels, respectively. 7.An apparatus as set forth in claim 1, 2, 3, 4, 5, or 6 furthercomprising a second focusing channel formed in the surface of thesubstrate for conducting a second focusing fluid stream therethrough,said second focusing channel having a first end in fluid communicationwith a source of focusing fluid and a second end in fluid communicationwith said chamber, said second end of said second focusing channel beingpositioned and arranged to provide said focusing fluid into said chambersuch that the sample fluid can be spatially confined in said chamber;and means for driving the focusing fluid in said first and secondfocusing channels such that said sample fluid is spatially confined insaid chamber.
 8. An apparatus as set forth in claim 7 wherein said meansfor driving the focusing fluid in said second focusing channel compriseselectromotive means for providing electrokinetic transport of thefocusing fluid through said second focusing channel.
 9. An apparatus asset forth in claim 7 wherein said focusing fluid driving means comprisesmeans for controlling the flow of the focusing fluid in said first andsecond focusing channels such that the sample fluid is spatiallyconfined substantially along the center axis of said chamber.
 10. Amethod of spatially confining a fluidic material in a channel comprisingthe steps of: providing a microfluidic device that includes a bodyhaving a first microchannel formed therein, said first microchannelbeing operatively connected to a source of a first material; providing afocusing channel in said microfluidic device having one end in fluidcommunication with a source of a focusing material and a second end influid communication with said first channel; transporting a stream ofthe first material through said first channel, said stream of firstmaterial having a width; and transporting a stream of the focusingmaterial through said focusing channel into said first channel such thatthe width of the stream of first material in said first channel isnarrowed.
 11. A method as set forth in claim 10 further comprising thesteps of providing a second microchannel formed in said body such thatsaid second microchannel communicates with said first channel at a firstintersection; and transporting the narrowed stream of the first materialthrough said second channel.
 12. A method as set forth in claim 10 or 11wherein the step of transporting the stream of the first materialthrough said first channel comprises the step of transporting the firstmaterial by electrokinetic transport means; and said step oftransporting the stream of the focusing material through said focusingchannel comprises the step of transporting the focusing material byelectrokinetic transport means.
 13. A method as set forth in claim 11wherein the step of transporting the narrowed stream of the firstmaterial through said second channel comprises the step of transportingthe narrowed stream of the first material by electrokinetic transportmeans.
 14. A method as set forth in claim 10 or 11 further comprisingthe steps of providing a second focusing channel in said microfluidicdevice having one end in fluid communication with a source of a focusingmaterial and a second end in fluid communication with said firstchannel; and transporting a stream of the focusing material through saidsecond focusing channel into said first channel such that the width ofthe stream of first material in said first channel is narrowed in saidfirst channel.
 15. A method as set forth in claim 14 comprising the stepof controlling the transport of the focusing material through saidfocusing channels such that the width of the stream of the firstmaterial is narrowed substantially along the center axis of said firstchannel.
 16. A method as set forth in claim 15 wherein said step oftransporting the streams of the focusing material through the focusingchannels comprises the step of transporting the focusing material byelectrokinetic transport means.
 17. A method of forming and dispensingminute volume segments of a material in a microfluidic device, saidmethod comprising the steps of: providing a microfluidic device thatincludes a body having first, second, third, and fourth microchannelsformed therein, said first, second, third, and fourth microchannelscommunicating at a first intersection and said first microchannel beingconnected to a source of a first material; providing a first focusingchannel in said body having one end in fluid communication with a sourceof a focusing material and a second end in fluid communication with saidfirst channel between said source of the first material and the firstintersection; transporting a stream of the first material through saidfirst channel toward second channel, said stream of first materialhaving a width; and transporting a stream of the focusing material fromthe first focusing channel into said first channel, such that the widthof the stream of first material in said first channel is narrowed.
 18. Amethod as set forth in claim 17 comprising the step of transporting abuffer material in said third and fourth channels such that said buffermaterial acts on said narrowed stream of the first material.
 19. Amethod as set forth in claim 17 or 18 comprising the step of dispensinga volume segment of the narrowed first material stream into the fourthchannel.
 20. A method as set forth in claim 19 wherein the step ofdispensing the volume segment of the narrowed first material stream intothe fourth channel comprises the steps of: transporting a first bufferfluid through said third channel toward the first intersection;transporting the volume segment of the narrowed first material streamthrough the fourth channel; and reversing the direction of the stream ofthe first material through said first channel.
 21. A method as set forthin claim 17 or 18 further comprising the steps of: providing a secondfocusing channel in said body having one end in fluid communication withthe focusing fluid and a second end in fluid communication with saidfirst channel between the source of the first material and the firstintersection; and transporting a second stream of the focusing materialfrom the second focusing channel into said first channel such that thewidth of the stream of the first material in said first channel isnarrowed substantially along the center axis of said first channel. 22.A method as set forth in claim 21 comprising the step of dispensing avolume segment of the narrowed first material stream into the fourthchannel.
 23. A method as set forth in claim 22 wherein the step ofdispensing the volume segment of the narrowed first material stream intothe fourth channel comprises the steps of: transporting a first bufferfluid through said third channel toward the first intersection;transporting the volume segment of the narrowed first material streamthrough the fourth channel; and reversing the direction of the stream ofthe first material through said first channel.
 24. An apparatus forforming and dispensing minute volume segments of a material in amicrofluidic device, the apparatus comprising: a substrate having first,second, third, and fourth microchannels formed therein, saidmicrochannels communicating at a first intersection and said firstmicrochannel being connected to a source of a first material; a focusingchannel formed in said substrate and having one end in fluidcommunication with a source of a focusing material and a second end influid communication with said first channel between said source of thefirst material and the first intersection; means for transporting astream of the first material through said first and second channels,said stream of first material having a width; means for transporting astream of the focusing material into said first channel, whereby thewidth of the stream of first material in said first channel is narrowed,and
 25. An apparatus as set forth in claim 24 further comprising meansfor transporting a buffer material in said third and fourth channelssuch that said buffer material acts on said narrowed stream of the firstmaterial.
 26. An apparatus as set forth in claim 24 or 25 comprisingmeans for dispensing a volume segment of the narrowed first materialstream into the fourth channel.
 27. An apparatus as set forth in claim26 wherein the means for dispensing the volume segment of the narrowedfirst material stream into the fourth channel comprises: means fortransporting a first buffer fluid through said third channel toward thefirst intersection; means for transporting the volume segment of thenarrowed first material stream through the fourth channel; and means forreversing the direction of the stream of the first material through saidfirst channel.
 28. An apparatus as set forth in claim 24 or 25 furthercomprising: a second focusing channel in said body having one end influid communication with the focusing fluid and a second end in fluidcommunication with said first channel between the source of the firstmaterial and the first intersection; and means for transporting a secondstream of the focusing material from the second focusing channel intosaid first channel such that the width of the stream of the firstmaterial in said first channel is narrowed substantially along thecenter axis of said first channel.
 29. An apparatus as set forth inclaim 28 comprising means for dispensing a volume segment of thenarrowed first material stream into the fourth channel.
 30. An apparatusas set forth in claim 22 wherein the means for dispensing the volumesegment of the narrowed first material stream into the fourth channelcomprises: means for transporting a first buffer fluid through saidthird channel toward the first intersection; means for transporting thevolume segment of the narrowed first material stream through the fourthchannel; and means for reversing the direction of the stream of thefirst material through said first channel.